Transformer consists of iron core (or core) and coil, the coil has two or more windings, which is connected to the power supply called the primary winding coil, the other called the secondary winding coil. It can transform AC voltage, current and impedance. The simplest core transformer consists of a core made of a soft magnetic material and two coils of different turns wound around the core, as shown in the figure.
The role of the core is to strengthen the magnetic coupling between the two coils. To reduce eddy currents and hysteresis loss in the iron, the core is made by lamination of painted silicon steel sheets; there is no electrical connection between the two coils, and the coil is made of insulated copper (or aluminum) wire. A coil connected to the AC power supply is called the primary coil (or the primary coil) and the other coil is called the secondary coil (or secondary coil). The actual transformer is very complex, there is inevitably copper loss (coil resistance fever), iron loss (core fever) and magnetic flux leakage (air-closed magnetic induction line), etc., in order to simplify the discussion here only describes the ideal transformer. Ideal transformer is established conditions are: ignore flux leakage, ignoring the original, the resistance of the secondary coil, ignoring the core loss, ignoring the no-load current (the secondary coil coil in the coil of the coil current). For example, the power transformer in full load operation (auxiliary coil output rated power) that is close to the ideal transformer situation.
Transformer is the use of electromagnetic induction principle made of static appliances. When the transformer primary coil connected to the AC power supply, the core will produce alternating flux, alternating flux φ said. The original, vice coil φ is the same, φ is also a simple harmonic function, the table is φ = φmsinωt. According to Faraday's law of electromagnetic induction, the induced electromotive force in the original coil and the auxiliary coil is e1 = -N1dφ / dt and e2 = -N2dφ / dt. Where N1, N2 is the original, the number of turns of the secondary coil. The figure shows that U1 = -e1 and U2 = e2 (the physical quantity of the primary coil is denoted by the lower subscript 1 and the physical quantity of the secondary coil is denoted by the lower subscript 2). The complex valid values are U1 = -E1 = jN1ωΦ and U2 = E2 = -jN2ωΦ, Let k = N1 / N2, said the transformer ratio. U1 / U2 = -N1 / N2 = -k can be obtained from the above equation, that is, the ratio of the effective voltage of the primary and secondary windings of the transformer is equal to the ratio of turns and the phase difference between the primary and secondary windings is π.
Then come to:
U1 / U2 = N1 / N2
In the no-load current can be ignored, I1 / I2 = -N2 / N1, that is, the primary and secondary coil current RMS value is inversely proportional to the number of turns, and the phase difference π.
And then available
I1 / I2 = N2 / N1
Ideal transformer original, the secondary coil power is equal P1 = P2. Describe the ideal transformer itself without power loss. The actual transformer total loss, its efficiency η = P2 / P1. Power transformer high efficiency, up to 90%. 
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